Aryl substituted monomers are useful in making liquid crystal polymers. Phenylhydroquinone, an example of one of these commercially useful monomers, can be prepared by alkylation of hydroquinone with cyclohexene, followed by dehydrogenation of the resulting cyclohexylhydroquinone over a noble metal catalyst at high temperatures. A considerable problem with this route is large yield losses which occur because of product and substrate decomposition under the severe conditions needed for the dehydrogenation of a saturated ring. Alternatively, cyclohexenylhydroquinone can be dehydrogenated under milder conditions which do not give large amounts of decomposition products.
D. A. Pisanenko and S. A. Nesterenko, Zh. Org. Khim, 15(9), 1979, describe the alkylation of hydroquinone with 1,3 cyclohexadiene using phosphoric acid catalyst to give cyclohexenylhydroquinone. This preparative method is severely limited by the tendency for 1,3 cyclohexadiene to react with itself in a disproportionation reaction to give benzene and cyclohexene. The cyclohexene thus formed then reacts with hydroquinone to give cyclohexyl hydroquinone rather than the desired cyclohexenylhydroquinone or, alternatively, reacts with another mole of cyclohexadiene to give benzene and cyclohexane. In addition, cyclohexadienes are expensive and availability is limited.